The X Factor judge is showing off her rockin' bod in the pages of October's Elle magazine.

Brit talks about not being camera-ready all the time .... unlike a famous reality starlet that we know quite well. She tells the mag:

"I love my jeans and my sweats – I'm really just a tomboy at heart. So it's really hard for me to be like Kim Kardashian and be makeup and hair ready every time I go out of my house. I'm not a believer in that, you know?"

As for her thoughts on fame, the 30-year-old songstress adds:

"I guess I've been under the microscope so long that I don't even pay attention to the nonsense anymore. I gave up getting upset about things people make up about me a long time ago."

The engaged singer also strikes a pose with her man Jason Trawick. Jason even opens up about the engagement, telling Elle:

"It was kind of one of those things where she moved, like, turned around, so we were face-to-face and then ..."